Notably, Janzen–Connell effects provide a recruitment advantage to locally-rare trees, since they act primarily on seeds and seedlings.
[3][4] The pest pressure hypothesis states that plant diversity is maintained by specialist natural enemies.
This lead both authors to predict that natural enemies contribute to the latitudinal diversity gradient by promoting local coexistence of many species in the warm, stable, highly productive climates of the wet tropics.
The black cherry is one such example of a temperate forest species whose growth patterns can be explained by the Janzen–Connell hypothesis.
Daniel Janzen published his hypothesis in 1970 in The American Naturalist under the article "Herbivores and the Number of Tree Species in Tropical Forests.
[2] Unlike Janzen, Connell proposed experiments that focused on the key prediction that exclusion of host-specific predators would cause a decrease in diversity as tree species with greater establishment or competitive ability formed low-diversity seedling and sapling communities where dominance was concentrated in a few species.
Along with Jack Greening Tracey and Larry Johnson Webb, he mapped trees in two rainforests and observed that smaller seedlings tended to occur in single-species clumps.
Through these observations, Connell suggests that each tree species has host-specific enemies that attack it and any of its offspring which are close to the parent.
Specialist herbivores who consume plant matter can also be thought of as having a "transmission rate" between individuals similarly to a disease.
When individuals are closer together at high density, movement between trees is easier and the predators quickly spread out.
Fewer studies assess the geographic implication that Janzen-Connell effects are stronger in the tropics.